This study was conducted to identify a potential CO2-storage-capacity-enhancement technique: enhancement of CO2 storage through co-injection or the simultaneous injection of CO2 and brine into saline aquifers. A series of triaxial permeability tests on brine-saturated sandstone samples for a range of injection pressures (4-10 MPa) under different confining pressures (20-35 MPa) was performed at 35 °C constant temperature. To clearly identify the sequestration-enhancement capability of the proposed novel method, conventional CO2 injection (injection of only CO2 into brine-saturated rock) was also carried out under the same test conditions. Both tests were subjected to comprehensive chemical analyses to evaluate the alteration of CO2 solubility in brine caused by the proposed method. CO2 flow behavior under conventional CO2 injection may convert from two- to single-phase flow over time around the injection point, as a result of forced brine migration. This may create a suddenly increased advective flux sometime after injection, which negatively affects the carbonation efficiency and eventually the mineral trapping process. Such issues can be eliminated by the proposed CO2-brine co-injection technique, because it offers significantly slower flow rates under the continuously available two-phase condition in the system. According to the results, the resulting slower flow rates under the co-injection process further enhance the mineral trapping process, allowing for additional time to activate the dissolution reaction compared to the conventional injection process. This will enable field projects to reduce the extensive time required for CO2 solubility in brine. Importantly, the solubility enhancement resulting from the proposed co-injection is much greater for supercritical CO2. This demonstrates the better performance expected of the proposed novel co-injection process under field conditions, because CO2 generally exists in its supercritical state in deep saline aquifers. In addition, the results show that the resulting H+ ion concentration increases with an increasing confining pressure, confirming the positive influence of aquifer depth on solubility under any injection conditions. Interestingly, this depth effect on solubility enhancement appears to be greater for co-injection than conventional injection. The proposed co-injection technique is also favorable for the long-term safety of the process, because the associated reduced CO2 relative permeability in saline aquifers also offers more opportunity for hydrodynamic trapping mechanisms.